I would never suggest that the royals can't do a good job sometimes, serve their country's armed forces, distinguish themselves through their actions in a myriad of different ways. They're human beings just like every other human beings in their own personal capacity after all. A lot of people sometimes do a good job. I wouldn't support putting them in line for the magic chair because of that. Someone would have to go way above and beyond the call of duty, exceed expectations by a significant degree, in order to earn that kind of privilege. Like Free Woman Ygritte said, to CHOOSE your king, not defend some $#@! king just because he's the son of the last one.
Cherry picking the good things the royals do is elementary. Thinking it earns them the privilege they receive is a whole other animal.
The point is that if the good things the royals did earn them the privilege they get through birthright, which seems to be the running assumption here that some of us are making, then a lot more people in British society would be born with the same privilege than currently are. Probably the vast majority of the British population. Just think of it! Millions upon millions of princes and sirs and lords! We'd have a lot of what Ms. Lovett only sang about: "And those up above will serve those down below!"